Thursday, July 19, 2007

Clues Boost 'Out of Africa' Theory

Twenty years after it was popularized, the "Out of Africa" theory, which posits that modern humans originally came from Africa before spreading out in a global conquest, has received an emphatic boost, scientists said on Wednesday.

Rival theories about the rise of Homo sapiens sapiens, as anatomically modern man is called, say humans either came from a single point in Africa or among different populations in different parts around the world, who evolved independently from a forebear, Homo erectus.

The "Out of Africa" scenario has been underpinned since 1987 by genetic studies based mainly on the rate of mutations in mitochondrial DNA, a genetic material inherited from the maternal line of ancestry.

The "multiple origins" school, meanwhile, points out that human skulls from around the world have clearly different characteristics, and argues that this proves our species evolved in slightly different forms more or less simultaneously.

In a study released by the British journal Nature, University of Cambridge researchers combined both techniques.

Analysis of genetic diversity among human populations is backed by evidence from 4,500 male skulls from around the world, demonstrating we all came from a single area in Africa, the authors say.

They found that the farther a population is from Africa, the less genetic diversity that population has.

This was the result of a "bottleneck," or interbreeding among a smaller gene pool that occurred when migrating populations were temporarily reduced by war, disease or some other catastrophe.

The loss in genetic diversity was mirrored by a corresponding loss in diversity of skull characteristics.

Applying a benchmark of characteristics, the researchers found that the most varied skulls were from southeastern Africa — and the diversity progressively declined the farther the skull was from Africa.

"We have combined our genetic data with new measurements of a large sample of skulls to show definitively that modern humans originated from a single area in sub-Saharan Africa," said lead researcher Andrea Manica of the university's Department of Zoology.

The team tested the "multiple origins" theory on these two tools, and found "this just did not work," said fellow researcher Francois Balloux.

In 2000, Swedish research based on the molecular clock estimated that H. sapiens sapiens emerged about 121,500 to 221,500 years ago, and the migration out of Africa was about 52,000 years ago, give or take 27,500 years.